The reaction of normal haemopoiesis under the development of a transferable acute leukaemia was studied in the rat. A reduction of cell numbers was observed in the bone marrow in the proliferating compartments of erythropoiesis, megakaryocytopoiesis and myelopoiesis. In the peripheral blood, erythrocyte and platelet levels decreased to one tenth of normal: Granulocytes and lymphocytes showed an increase coincident with the increase in circulating leukaemic cells before a final tendency to decline. These changes are attributed partly to variations in the inflow from the bone marrow and partly to direct effects on the peripheral blood cells themselves. The reduced number of cells in the proliferation compartment may be brought about by effects on this compartment itself, but there are some indications that a reduced inflow from the preceding stem cell compartment might be responsible.